Photoluminescent materials and compositions that contain photoluminescent phosphorescent materials with emissions in the visible region of the electromagnetic spectrum have been disclosed. For example, metal sulfide pigments which contain various elemental activators, co-activators and compensators have been prepared which absorb at 380-400 nm and have an emission spectrum of 450-520 nm.
Recently, phosphorescent materials that have significantly higher persistence, up to 12-16 hours, have been reported. Such phosphors generally comprise an alkaline earth aluminate host matrix and can be represented, for example, by MAl2O3 or MAl2O4 wherein M can comprise a plurality of metals, at least one of which is an alkaline earth metal, such as calcium, strontium, barium and magnesium. These materials generally deploy Europium as an activator and can additionally also contain one or more rare earth materials as co-activators. Examples of such high intensity and high persistence phosphors can be found, for example, in U.S. Pat. No. 5,424,006, U.S. Pat. No. 5,885,483, U.S. Pat. No. 6,117,362 and U.S. Pat. No. 6,267,911 B1.
Photoluminescent compositions comprising only phosphorescent materials with emissions in the infrared region have been reported. Such phosphorescent materials consist of doped ZnCdS. These materials have been shown to have observable tail emissions into the visible region and consequently would not have utility for clandestine markings. These materials have neither been used for clandestine detection or for detection applications wherein activation and detection can be decoupled spatially and temporally.
Photoluminescent compositions which contain combinations of ZnS phosphorescent materials and fluorescent materials have also been disclosed. However the use of these fluorescent materials has been limited to either altering the charging (activating) radiation or altering the visible daylight or visible emission color. Use of ZnS with fluorescent materials is generally limited to altering the color observed in daylight.
Photoluminescent compositions have also been contemplated which contain a series of fluorescent materials. One of the fluorescent materials absorbs and emits radiation which is then absorbed by a companion fluorescent material which then emits radiation to give a final infrared emission. As can be appreciated, use of fluorescent materials does not provide for any continued emission once the absorbable radiation is removed. These compositions have no provision for continued emission of infrared radiation that can be detected at a future time, that is, after activation has ceased. The need for activating the materials immediately prior to detection will also require possession of activating equipment at site of detection thereby limiting flexibility and/or portability and thus will not permit stealth detection.
Prior art in the field of photoluminescent fibers has focused entirely on visible emission such as U.S. Pat. No. 5,674,437; U.S. Pat. No. 6,162,539, U.S. Pat. No. 7,338,877 and U.S. Pat. No. 6,307,207B1. No work has been reported for creating photoluminescent fibers with emissions partly or fully in the infrared region of the electromagnetic spectrum.
Although methods for uniquely marking and identifying objects have received thought and attention, such methods do not enable stealth detection. In many cases, such as, for example, identification of friendly forces in the combat theater, the identifying markings need to be unobservable by enemy personnel, or, for example, in anti-counterfeit applications wherein, the identifying markings need to be hidden to avoid detectability of such markings by counterfeiters. Clandestine or stealth identification, wherein the emissions from the photoluminescent markings are not ordinarily observable by a human observer (without specific detection equipment), but detectable by friendly forces, and further wherein activation is not required during detection (such activation being potentially detectable by others), will be of high value in the combat theater for stealth detection of combat equipment, or personnel. Such markings will also be of value for stealth combat operations, or for covertly marking enemy targets for tracking or elimination.
There is no mention in the literature of fibers that photoluminesce fully or partially in the infrared region of the electromagnetic spectrum. Therefore there are no fabrics which contain these fibers that could be used for clandestine or stealth identification, or where these fibers could be used for authentication purposes.
As can be seen from the above discussion, there is a need for photoluminescent fibers, fabrics made therefrom and objects containing the fiber which emit partly or fully in the infrared region of the electromagnetic spectrum useful for identification and detection of objects. Furthermore there is also a need for photoluminescent fibers, fabrics made therefrom and objects containing the fiber that enable the act of detection of the object to be decoupled spatially from the object and/or its activation source, that is, detection can occur away from the object and/or its activation source, and also wherein, detection can be decoupled temporally from activation, that is, detection can occur at a time later than the activation. It should be noted that decoupling of activation and detection also allows for flexibility and portability in the act of detection, allowing for clandestine or stealth identification and detection.